Series-connected transistor structure

ABSTRACT

A series-connected transistor structure includes a first source, a first channel-drain structure, a second channel-drain structure, a gate dielectric layer, a gate, a first drain pad and a second drain pad. The first source is over a substrate. The first channel-drain structure is over the first source and includes a first channel and a first drain thereover. The second channel-drain structure is over the first source and substantially parallel to the first channel-drain structure and includes a second channel and a second drain thereover. The gate dielectric layer surrounds the first channel and the second channel. The gate surrounds the gate dielectric layer. The first drain pad is over and in contact with the first drain. The second drain pad is over and in contact with the second drain, in which the first drain pad and the second drain pad are separated from each other.

RELATED APPLICATIONS

The present application is a Divisional Application of the U.S.application Ser. No. 14/485,541, filed Sep. 12, 2014.

BACKGROUND

Semiconductor devices may be classified into I/O devices and coredevices in accordance with functions thereof, in which both the I/Odevices and core devices generally include horizontal transistors. Sincehigher voltage is applied to the I/O device compared to the core device,a length of a gate and a thickness of a gate dielectric layer of thehorizontal transistor of the I/O device should be much greater thanthose of the core device, which results in great occupied area and poorintegration density.

In order to improve integration density, vertical transistors with thesame gate length in the I/O devices and the core devices may be applied.However, gate length and channel length of the vertical transistor aremuch less than that of the horizontal transistor, such that draininduced barrier lowering (DIBL) and hot carrier injection (HCl)phenomena may occur in the vertical transistor of the I/O device whenhigh voltage is applied, and thus to increase leakage current andsignificantly reduce reliability. Accordingly, improvements in thevertical transistor of the I/O device continue to be sought.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.

FIG. 2 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.

FIG. 3 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.

FIG. 4 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.

FIG. 5 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.

FIGS. 6A-6F are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure.

FIGS. 7A-7G are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure.

FIGS. 8A-8G are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

As aforementioned, when high voltage is applied to the verticaltransistor, DIBL and HCI phenomena may occur and thus to increaseleakage current and significantly reduce reliability. To address theabove issue, a series-connected transistor structure for sharing voltage(e.g., drain-to-source voltage (Vds)) is provided to effectively reduceor prevent DIBL and HCl phenomena, and thus to decrease leakage currentand improve reliability. Embodiments of the series-connected transistorstructures and methods for manufacturing the same will be sequentiallydescribed below in detail.

FIG. 1 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.The series-connected transistor structure includes a first source S1, afirst channel-drain structure CDS1, a second channel-drain structureCDS2, a gate dielectric layer 130, a gate G, a first drain pad DP1 and asecond drain pad DP2. In some embodiments, the series-connectedtransistor structure is in an I/O device. In some embodiments, theseries-connected transistor structure is in a core device.

The first source S1 is over a substrate 110. In some embodiments, thesubstrate 110 includes an elementary semiconductor including silicon orgermanium in crystal, polycrystalline, or an amorphous structure; acompound semiconductor including silicon carbide, gallium arsenide,gallium phosphide, indium phosphide, indium arsenide, and indiumantimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and GaInAsP; any other suitable material; orcombinations thereof. In some embodiments, the substrate 110 includes awell region (not shown) of a conductivity type different from that ofthe first source S1 extending into the substrate 110 from an uppersurface thereof. In some embodiments, the first source S1 is on and incontact with the well region. In some embodiments, the substrate 110 hasan I/O region 110 a and a core region (not shown), and the first sourceS1 is over the I/O region 110 a of the substrate 110.

In some embodiments, the first source S1 is a heavily doped layer of aconductivity type. In some embodiments, the first source S1 is a heavilyn-doped layer. In some embodiments that the first source S1 is theheavily n-doped layer, the well region is a p-type well region. In someembodiments, the first source S1 includes n-type dopants, such asphosphorus, arsenic, antimony, bismuth, selenium, tellurium, anothersuitable n-type dopants or a combination thereof. In some embodiments,the series-connected transistor structure further includes a sourcesilicide region SSR in the first source S1 to reduce resistance thereof.In other words, the source silicide region SSR can be regarded as aportion of the first source S1. In some embodiments, the first source S1includes a plurality of source silicide regions SSR spaced apart.

The first channel-drain structure CDS1 is over the first source S1. Thefirst channel-drain structure CDS1 includes a first channel C1 and afirst drain D1 over the first channel C1. In some embodiments, the firstchannel C1 is a doped layer of the same conductivity type as the firstsource S1. In some embodiments, the first drain D1 is a heavily dopedlayer of the same conductivity type as the first source S1.

The second channel-drain structure CDS2 is also over the first source S1and substantially parallel to the first channel-drain structure CDS1.The second channel-drain structure CDS2 includes a second channel C2 anda second drain D2 over the second channel C2. In some embodiments, thesecond channel C2 is a doped layer of the same conductivity type as thefirst source S1. In some embodiments, the second drain D2 is a heavilydoped layer of the same conductivity type as the first source S1. Insome embodiments, the first channel-drain structure CDS1 and the secondchannel-drain structure CDS2 are vertical nanowire structures.

The gate dielectric layer 130 surrounds the first channel C1 and thesecond channel C2. In some embodiments, the gate dielectric layer 130includes a dielectric material such as silicon dioxide, silicon nitride,silicon oxynitride or another suitable insulating material. The gate Gsurrounds the gate dielectric layer 130. In other words, theseries-connected transistor structure belongs to a verticalgate-all-around (VGAA) transistor. In some embodiments, the gate Gincludes a conductive material such as polysilicon (poly), metal ormetal alloy.

The first drain pad DP1 is over and in contact with the first drain D1,and the second drain pad DP2 is over and in contact with the seconddrain D2. The first drain pad DP1 and the second drain pad DP2 areseparated from each other. In some embodiments, the first drain pad DP1and the second drain pad DP2 include metal, silicide or other conductivematerials. When high voltage is applied to the first drain pad DP1 orthe second drain pad DP2, it will be shared to the first channel-drainstructure CDS1 and the second channel-drain structure CDS2 through thefirst source S1, as shown in dotted lines of FIG. 1. In someembodiments, the series-connected transistor structure includes aplurality of the first channel-drain structures CDS1 and a plurality ofthe second channel-drain structures CDS2, and the first drain pad DP1 isover and in contact with the first drains D1, and the second drain padDP2 is over and in contact with the second drains D2. When high voltageis applied to the first drain pad DP1 or the second drain pad DP2, itwill be shared to the first channel-drain structures CDS1 and the secondchannel-drain structures CDS2 through the first source S1. In someembodiments, the first channel-drain structures CDS1 and the secondchannel-drain structures CDS2 are vertical nanowire structures.

In some embodiments, the series-connected transistor structure furtherincludes a source dielectric layer 140 between the first source S1 andthe gate G to electrically isolate the first source S1 from the gate G.In some embodiments, the source dielectric layer 140 includes adielectric material such as silicon dioxide, silicon nitride, siliconoxynitride or another suitable insulating material.

In some embodiments, the series-connected transistor structure furtherincludes a high-k dielectric layer 150 between the gate dielectric layer130 and the gate G and between the source dielectric layer 140 and thegate G. In some embodiments, the high-k dielectric layer 150 includesHfO₂, ZrO₂, Y₂O₃, La₂O₅, Gd₂O₅, TiO₂, Ta₂O₅, HfErO, HfLaO, HfYO, HfGdO,HfAlO, HfZrO, HfTiO, HfTaO, SrTiO, or combinations thereof.

In some embodiments, the series-connected transistor structure furtherincludes an inter-layer dielectric (ILD) 160 covering the gate G andexposing an upper surface of the first drain D1 and an upper surface ofthe second drain D2. The first drain pad DP1 and the second drain padDP2 are over the ILD 160 and respectively in contact with the firstdrain D1 and the second drain D2. In some embodiments, theseries-connected transistor structure further includes another ILD 210over the first drain pad DP1 and the second drain pad DP2. In someembodiments, the ILDs 160, 210 are made of silicon oxide, siliconoxynitride and/or other suitable insulating material. In someembodiments, the series-connected transistor structure further includestwo conductive plugs P through the ILD 210 and respectively connected tothe first drain pad DPI and the second drain pad DP2. In someembodiments, the conductive plug P includes metal, metal compound or acombination thereof, such as Ti, Ta, W, Al, Cu, Mo, Pt, TiN, TaN, TaC,TaSiN, WN, MoN, MOON, RuO₂, TiAl, TiAlN, TaCN, combinations thereof orother suitable materials.

FIG. 2 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.The series-connected transistor structure includes a first source S1, asecond source S2, an isolation portion 120, a second channel-drainstructure CDS2, a third channel-drain structure CDS3, a gate dielectriclayer 130, a gate G and a second drain pad DP2. In some embodiments, theseries-connected transistor structure is in an I/O device. In someembodiments, the series-connected transistor structure is in a coredevice.

The first source S1 is over a substrate 110. The second source S2 isalso over the substrate 110 and laterally adjacent to the first sourceS1. In some embodiments, the substrate 110 has an I/O region 110 a and acore region (not shown), and the first source S1 and the second sourceS2 are over the I/O region 110 a of the substrate 110. In someembodiments, the first source S1 and the second source S2 are heavilydoped layers of the same conductivity type. In some embodiments, thefirst source S1 and the second source S2 are heavily n-doped layers. Insome embodiments, the first source S1 and the second source S2 are madeof a same layer. In some embodiments, the series-connected transistorstructure further includes a plurality of source silicide regions SSR inthe first source S1 and the second source S2.

The isolation portion 120 is between the first source S1 and the secondsource S2 to electrically isolate the first source S1 from the secondsource S2. In some embodiments, the isolation portion 120 is a shallowtrench isolation (STI). In some embodiments, the isolation portion 120includes silicon oxide, silicon nitride, silicon oxynitride, a low-kdielectric material, and/or a combination thereof.

The second channel-drain structure CDS2 is over the first source S1. Thesecond channel-drain structure CDS2 includes a second channel C2 and asecond drain D2 over the second channel C2. In some embodiments, thesecond channel C2 is a doped layer of the same conductivity type as thefirst source S1. In some embodiments, the second drain D2 is a heavilydoped layer of the same conductivity type as the first source S1.

The third channel-drain structure CDS3 is over the second source S2 andsubstantially parallel to the second channel-drain structure CDS2. Thethird channel-drain structure CDS3 includes a third channel C3 and athird drain D3 over the third channel C3. In some embodiments, the thirdchannel C3 is a doped layer of the same conductivity type as the secondsource S2. In some embodiments, the third drain D3 is a heavily dopedlayer of the same conductivity type as the second source S2. In someembodiments, the second channel-drain structure CDS2 and the thirdchannel-drain structure CDS3 are vertical nanowire structures.

The gate dielectric layer 130 surrounds the second channel C2 and thethird channel C3. In some embodiments, the gate dielectric layer 130includes a dielectric material such as silicon dioxide, silicon nitride,silicon oxynitride or another suitable insulating material. The gate Gsurrounds the gate dielectric layer 130. In some embodiments, the gate Gincludes a conductive material such as polysilicon (poly), metal ormetal alloy.

The second drain pad DP2 is over and in contact with the second drain D2and the third drain D3. In some embodiments, the second drain pad DP2includes metal, silicide, or other conductive materials. Silicide may becobalt silicide, titanium silicide, tungsten silicide, nickel silicideor a combination thereof. When high voltage is applied to the firstsource S1 or the second source S2, it will be shared to the secondchannel-drain structure CDS2 and the third channel-drain structure CDS3through the second drain pad DP2, as shown in dotted lines of FIG. 2. Insome embodiments, the series-connected transistor structure includes aplurality of the second channel-drain structures CDS2 over the firstsource S1 and a plurality of the third channel-drain structures CDS3over the second source S2, and the second drain pad DP2 is over and incontact with the second drains D2 and the third drains D3. When highvoltage is applied to the first source S1 or the second source S2, itwill be shared to the second channel-drain structures CDS2 and the thirdchannel-drain structures CDS3 through the second drain pad DP2. In someembodiments, the second channel-drain structures CDS2 and the thirdchannel-drain structures CDS3 are vertical nanowire structures.

In some embodiments, the series-connected transistor structure furtherincludes a source dielectric layer 140 between the first source S1 andthe gate G and between the second source S2 and the gate G. In someembodiments, the source dielectric layer 140 includes a dielectricmaterial such as silicon dioxide, silicon nitride, silicon oxynitride oranother suitable insulating material.

In some embodiments, the series-connected transistor structure furtherincludes a high-k dielectric layer 150 between the gate dielectric layer130 and the gate G and between the source dielectric layer 140 and thegate G. In some embodiments, the high-k dielectric layer 150 includesHfO₂, ZrO₂, Y₂O₃, La₂O₅, Gd₂O₅, TiO₂, Ta₂O₅, HfErO, HfLaO, HfYO, HfGdO,HfAlO, HfZrO, HfTiO, HfTaO, SrTiO, or combinations thereof.

In some embodiments, the series-connected transistor structure furtherincludes an ILD 160 covering the gate G and exposing an upper surface ofthe second drain D2 and an upper surface of the third drain D3. Thesecond drain pad DP2 is over the ILD 160 and in contact with the seconddrain D2 and the third drain D3. In some embodiments, theseries-connected transistor structure further includes another ILD 210over the second drain pad DP2. In some embodiments, the ILDs 160, 210are made of silicon oxide, silicon oxynitride and/or other suitableinsulating material. In some embodiments, the series-connectedtransistor structure further includes two conductive plugs P through theILDs 210, 160 and respectively connected to the first source S1 and thesecond source S2. In some embodiments, the conductive plug P includesmetal, metal compound or a combination thereof.

FIG. 3 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.The difference between the series-connected transistor structures ofFIG. 2 and FIG. 3 is that the second drain pad DP2 of FIG. 3 includes afirst drain pad layer DPL1 and a second drain pad layer DPL2. The firstdrain pad layer DPL1 is over and in contact with the second drain D2 andthe third drain D3 and includes silicide. The second drain pad layerDPL2 is over the first drain pad layer DPL1 and includes metal, metalcompound or a combination thereof to further reduce resistance of thesecond drain pad DP2. The metal or metal compound may be Ti, Ta, W, Al,Cu, Mo, Pt, TiN, TaN, TaC, TaSiN, WN, MoN, MOON, RuO₂, TiAl, TiAlN,TaCN, combinations thereof or other suitable materials. In someembodiments, the conductive plug P and the second drain pad layer DPL2are made of the same material.

FIG. 4 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.The difference between the series-connected transistor structures ofFIG. 4 and FIG. 2 is that the series-connected transistor structure ofFIG. 4 further includes a first channel-drain structure CDS1 and a firstdrain pad DP1, and two conductive plugs P are respectively connected tothe first drain pad DP1 and the second source S2.

The first channel-drain structure CDS1 is over the first source S1 andsubstantially parallel to the second channel-drain structure CDS2. Thefirst channel-drain structure CDS1 includes a first channel C1 and afirst drain D1 over the first channel C1. The gate dielectric layer 130further surrounds the first channel C1. The first drain pad DP1 is overand in contact with the first drain D1. The first drain pad DP1 and thesecond drain pad DP2 are separated from each other. When high voltage isapplied to the first drain pad DP1 or the second source S2 through oneof the conductive plugs P, it will be shared to the first channel-drainstructure CDS1, the second channel-drain structure CDS2 and the thirdchannel-drain structure CDS3 through the first source S1 and the seconddrain pad D2, as shown in dotted lines of FIG. 4. In other embodiments,the series-connected transistor further includes a fourth channel-drainstructure (not shown) over the second source S2 to replace theconductive plug P connected to the second source S2.

FIG. 5 is a cross-sectional view of a series-connected transistorstructure in accordance with some embodiments of the present disclosure.The series-connected transistor structure includes a first source S1, asecond source S2, an isolation portion 120, a plurality of firstchannel-drain structure CDS1, a plurality of second channel-drainstructures CDS2, a plurality of third channel-drain structures CDS3, aplurality of fourth channel-drain structures CDS4, a gate dielectriclayer 130, a gate G, a first drain pad DP1, a second drain pad DP2 and athird drain pad DP3. In some embodiments, the series-connectedtransistor structure further includes a first conductive plug P1, asecond conductive plug P2 and a third conductive plug P3. The firstconductive plug P1 is connected to the gate G. The second conductiveplug P2 is connected to the first drain pad DP1. The third conductiveplug P3 is connected to the third drain pad DP3. In some embodiments, Vgis applied on the gate G through the first conductive plug P1, and Vddis applied on the first drain pad DP1 through the second conductive plugP2, and the third conductive plug P3 is electrically connected to aground potential. The series-connected transistor structure can be usedto provide high gain.

It is noteworthy that the series-connected transistor structure of thepresent disclosure may have a smaller occupied area than that of aseries-connected FinFET structure since the series-connected transistorstructure of the present disclosure can be considered as a verticalfolding structure, and the series-connected FinFET structure is not afolding structure. In another point of view, in the case of the sameoccupied area, the series-connected transistor structure has higher gainthan that of the typical series-connected FinFET structure.

In addition, it is found that the series-connected transistor structurehas low local variation of Vth (i.e., threshold voltage) compared to aparallel-connected transistor structure. The difference between theseries-connected transistor structure and the parallel-connectedtransistor structure is that for the parallel-connected transistorstructure, there is only one drain pad connected to drains ofchannel-drain structures, and there is only one source connected tochannels thereof.

As mentioned above, the series-connected transistor structure of thepresent disclosure can be used to share Vds by two or more channel-drainstructures, one or more sources and one or more drain pads toeffectively reduce or prevent DIBL and HCl phenomena. In addition, theseries-connected transistor structure of the present disclosure occupiesa small area compared to a horizontal transistor or a series-connectedtransistor structure connected through metal lines and conductive plugs,so as to have higher integration density.

FIGS. 6A-6F are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure. As shown in FIG. 6A, a substrate110 is provided. In some embodiments, the substrate 110 includes anelementary semiconductor, a compound semiconductor, an alloysemiconductor or combinations thereof. In some embodiments, a wellimplant process is performed to form a well region (not shown) of aconductivity type extending into the substrate 110 from an upper surfacethereof.

A source layer SL, a channel layer CL and a drain layer DL are thensequentially formed over the substrate 110, as shown in FIG. 6A. In someembodiments, the source layer SL, the channel layer CL and the drainlayer DL are sequentially formed by epitaxial (epi) growth and dopingprocesses with different dopant concentrations. In some embodiments, thesource layer SL, the channel layer CL and the drain layer DL are formedusing ion implantation and annealing processes. A dopant concentrationof the channel layer CL is lower than that of the source layer SL or thedrain layer DL.

Subsequently, a hard mask layer HM is formed over the drain layer DL forpatterning the drain layer DL, the channel layer CL and the source layerSL, as shown in FIG. 6A. In some embodiments, a hard mask material isformed using a chemical vapor deposition (CVD) process, a physical vapordeposition (PVD) process, spin-on coating, or another suitable formationprocess, and then patterned using a photolithography process or anothersuitable material removal process to form the hard mask layer HM.

As shown in FIGS. 6A-6B, the drain layer DL, the channel layer CL andthe source layer SL are patterned according to the hard mask layer HM toform a series-connected source-channel-drain structure protruding overthe substrate 110, which includes a first source S1, a firstchannel-drain structure CDS1 and a second channel-drain structure CDS2.The first channel-drain structure CDS1 and a second channel-drainstructure CDS2 are over the first source S1 and substantially parallelto each other. The first channel-drain structure CDS1 includes a firstchannel C1 and a first drain D1 over the first channel C1, and thesecond channel-drain structure CDS2 includes a second channel C2 and asecond drain D2 over the second channel C2. In some embodiments, asshown in FIG. 6A, the drain layer DL exposed from the hard mask HM andthe channel layer CL and the source layer SL therebeneath are removed bya dry etching process. In some embodiments, the etchant includes carbonfluorides (C_(x)F_(y)), sulfur hexafluoride (SF₆), oxygen gas (O₂),helium (He), carbon chlorides (C_(x)Cl_(y)), argon (Ar) or anothersuitable etchant material.

As shown in FIG. 6C, a plurality of source silicide regions SSR areformed in the first source S1. The source silicide regions SSR can beused to reduce resistance of the first source S1. In some embodiments,the source silicide regions SSR are formed by silicide depositionprocess. In some embodiments, the source silicide regions SSR are formedusing metal deposition and annealing processes. In some embodiments,there is no source silicide region formed in the first source S1.

As shown in FIG. 6D, a source dielectric layer 140 is formed coveringthe first source S1 and the source silicide regions SSR. In someembodiments, the source dielectric layer 140 is formed using a CVDprocess, a PVD process, a spin-on coating process or another suitableformation process.

A gate dielectric layer 130 is then formed surrounding the first channelC1 and the second channel C2, as shown in FIG. 6D. In some embodiments,the gate dielectric layer 130 is formed using a PVD process, a CVDprocess, a thermal wet oxidation, a thermal dry oxidation, a thermalplasma oxidation or another formation process. In some embodiments, thegate dielectric layer 130 is formed using a thermal oxidation process.In some embodiments, a dielectric layer (not shown) is previously formedfully covering the first drain D1 and the second drain D2 to preventoxidation during the thermal oxidation process for forming the gatedielectric layer 130.

As shown in FIG. 6E, a high-k dielectric layer 150 and a gate G areformed over the source dielectric layer 140 and surrounding the gatedielectric layer 130. The source dielectric layer 140 is used toelectrically isolate the first source S1 from the gate G. In someembodiments, a high-k dielectric material and a gate material aresequentially blanket deposited and then patterned to form the high-kdielectric layer 150 and the gate G. In some embodiments, the high-kdielectric material is blanket deposited using a CVD process, an ALD(atomic layer deposition) process or another suitable formation process.In some embodiments, the gate material is formed using a PVD process, aCVD process, an ALD process, a plating process, a spin-on coatingprocess or another suitable formation process. In some embodiments, thegate material and the high-k dielectric material are patterned using aphotolithography/etching process or another suitable material removalprocess.

As shown in FIG. 6F, an ILD 160 is formed covering the firstchannel-drain structure CDS1, the second channel-drain structure CDS2and the gate G, and a planarization process is then performed to exposean upper surface of the first drain D1 and an upper surface of thesecond drain D2. In some embodiments, the ILD 160 is formed using a CVDprocess, a PVD process, an ALD process, a spin-on process or anothersuitable formation process. In some embodiments, the planarizationprocess includes a chemical mechanical polishing (CMP) process, agrinding process, an etching process or another suitable materialremoval process. In some embodiments, after the planarization process,an upper surface of the first drain D1 and an upper surface of thesecond drain D2 are coplanar with an upper surface of the ILD 160.

Subsequently, a first drain pad DP1 and a second drain pad DP2 areformed respectively over and in contact with the first drain D1 and thesecond drain D2, as shown in FIG. 6F. In some embodiments, a drain padmaterial is formed using any suitable formation process and thenpatterned using a photolithography/etching process or another suitablematerial removal process to form the first drain pad DP1 and the seconddrain pad DP2. In some embodiments, the first drain pad DP1 and thesecond drain pad DP2 include metal, silicide or a combination thereof.

After the formation of the first drain pad DP1 and the second drain padDP2, another ILD 210 is formed over the first drain pad DP1, the seconddrain pad DP2 and the ILD 160, as shown in FIG. 1. In some embodiments,the ILD 210 is formed using a CVD process, a PVD process, an ALDprocess, a spin-on process or another suitable formation process.

Afterwards, the ILD 210 is patterned to form openings, and a conductivematerial is then filled in the openings to form conductive plugs Prespectively connected to the first drain pad DP1 and the second drainpad DP2. In some embodiments, the ILD 210 is patterned using aphotolithography/etching process, a laser drilling process or anothersuitable material removal process. In some embodiments, the conductivematerial is formed using a CVD process, a PVD process, an ALD process, aspin-on coating process or another formation process.

FIGS. 7A-7G are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure. As shown in FIG. 7A, a substrate110 having an isolation portion 120 over the substrate 110 is provided.The isolation portion 120 is formed extending from inside the substrate110 to outside the substrate 110. In some embodiments, a thick substrate(not shown) is provided, and the isolation portion 120 is formedtherein. The thick substrate is then thinned to form the substrate 110having the isolation portion 120. In some embodiments, before the thicksubstrate is thinned, a well implant process is performed to form a wellregion (not shown) of a conductivity type extending into the thicksubstrate from an upper surface thereof.

As shown in FIG. 7B, a source layer SL, a channel layer CL and a drainlayer DL are then sequentially formed over the substrate 110 andadjacent to the isolation portion 120, as shown in FIG. 7B. In someembodiments, the source layer SL, the channel layer CL and the drainlayer DL are sequentially formed by epi growth process and dopingprocesses with different dopant concentrations. In some embodiments, thesource layer SL, the channel layer CL and the drain layer DL are formedusing ion implantation and annealing processes.

Subsequently, a hard mask layer HM is formed over the drain layer DL forpatterning the source layer SL, the channel layer CL and the drain layerDL, as shown in FIG. 7B. In some embodiments, a hard mask material isformed using a CVD process, a PVD process, spin-on coating, or anothersuitable formation process, and then patterned using a photolithographyprocess or another suitable material removal process to form the hardmask layer HM.

As shown in FIGS. 7B-7C, the drain layer DL, the channel layer CL andthe source layer SL are patterned according to the hard mask layer HM toform a series-connected source-channel-drain structure protruding overthe substrate 110, which includes a first source S1, a second source S2,a second channel-drain structure CDS2 over the first source S1 and athird channel-drain structure CDS3 over the second source S2. Theisolation portion 120 is between the first source S1 and the secondsource S2. The second channel-drain structure CDS2 and the thirdchannel-drain structure CDS3 are substantially parallel to each other.The second channel-drain structure CDS2 includes a second channel C2 anda second drain D2 over the second channel C2, and the thirdchannel-drain structure CDS3 includes a third channel C3 and a thirddrain D3 over the third channel C3. In some embodiments, as shown inFIG. 7B, the drain layer DL exposed from the hard mask HM and thechannel layer CL and the source layer SL therebeneath are removed by adry etching process.

As shown in FIG. 7D, a plurality of source silicide regions SSR areformed in the first source S1 and the second source S2. The sourcesilicide regions SSR can be used to reduce resistance of the firstsource S1 and that of the second source S2. In some embodiments, thesource silicide regions SSR are formed by silicide deposition process.In some embodiments, the source silicide regions SSR are formed usingmetal deposition and annealing processes. In some embodiments, there isno source silicide region formed in the first source S1 and the secondsource S2.

As shown in FIG. 7E, a source dielectric layer 140 is formed coveringthe first source S1, the second source S2, the source silicide regionsSSR and the isolation portion 120. In some embodiments, the sourcedielectric layer 140 is formed using a CVD process, a PVD process, aspin-on coating process or another suitable formation process.

A gate dielectric layer 130 is then formed surrounding the secondchannel C2 and the third channel C3, as shown in FIG. 7E. In someembodiments, the gate dielectric layer 130 is formed using a PVDprocess, a CVD process, a thermal wet oxidation, a thermal dryoxidation, a thermal plasma oxidation or another formation process. Insome embodiments, the gate dielectric layer 130 is formed using athermal oxidation process. In some embodiments, a dielectric layer (notshown) is previously formed fully covering the second drain D2 and thethird drain D3 to prevent oxidation during the thermal oxidation processfor forming the gate dielectric layer 130.

As shown in FIG. 7F, a high-k dielectric layer 150 and a gate G areformed over the source dielectric layer 140 and surrounding the gatedielectric layer 130. In some embodiments, a high-k dielectric materialand a gate material are sequentially blanket deposited and thenpatterned using a photolithography/etching process to form the high-kdielectric layer 150 and the gate G.

As shown in FIG. 7G, an ILD 160 is formed covering the secondchannel-drain structure CDS2, the third channel-drain structure CDS3 andthe gate G, and a planarization process is then performed to expose anupper surface of the second drain D2 and an upper surface of the thirddrain D3. In some embodiments, the ILD 160 is formed using a CVDprocess, a PVD process, an ALD process, a spin-on process or anothersuitable formation process. In some embodiments, the planarizationprocess includes a CMP process, a grinding process, an etching processor another suitable material removal process. In some embodiments, afterthe planarization process, an upper surface of the second drain D2 andan upper surface of the third drain D3 are coplanar with an uppersurface of the ILD 160.

Subsequently, a second drain pad DP2 is formed over and in contact withthe second drain D2 and the third drain D3, as shown in FIG. 7G. In someembodiments, a drain pad material is formed using any suitable formationprocess and then patterned using a photolithography/etching process oranother suitable material removal process to form the second drain padDP2. In some embodiments, the second drain pad DP2 includes metal,silicide or a combination thereof.

After the formation of the second drain pad DP2, another ILD 210 isformed over the second drain pad DP2 and the ILD 160, as shown in FIG.2. In some embodiments, the ILD 210 is formed using a CVD process, a PVDprocess, an ALD process, a spin-on process or another suitable formationprocess.

Afterwards, the ILDs 210, 160 are patterned to form openings, and aconductive material is then filled in the openings to form conductiveplugs P respectively connected to the first source S1 and the secondsource S2. In some embodiments, the ILDs 210, 160 are patterned using aphotolithography/etching process, a laser drilling process or anothersuitable material removal process. In some embodiments, the conductivematerial is formed using a CVD process, a PVD process, an ALD process, aspin-on coating process or another formation process.

FIGS. 8A-8G are cross-sectional views at various stages of manufacturinga series-connected transistor structure in accordance with someembodiments of the present disclosure. As shown in FIG. 8A, a substrate110 having an isolation portion 120 over the substrate 110 is provided.The isolation portion 120 is formed extending inside the substrate 110to outside the substrate 110. As shown in FIG. 8B, a source layer SL, achannel layer CL and a drain layer DL are then sequentially formed overthe substrate 110 and adjacent to the isolation portion 120.Subsequently, a hard mask layer HM is formed over the drain layer DL forpatterning the drain layer DL, the channel layer CL and the source layerSL.

As shown in FIGS. 8B-8C, the drain layer DL, the channel layer CL andthe source layer SL are patterned according to the hard mask layer HM toform a series-connected source-channel-drain structure protruding overthe substrate 110, which includes a first source S1, a second source S2,a first channel-drain structure CDS1 and a second channel-drainstructure CDS2 over the first source S1, and a third channel-drainstructure CDS3 over the second source S2. That is, compared to theembodiments of FIGS. 7B-7C, the embodiments of FIGS. 8B-8C furtherinclude forming the first channel-drain structure CDS1 over the firstsource S1 and substantially parallel to the second channel-drainstructure CDS2. The first channel-drain structure CDS1 includes a firstchannel C1 and a first drain D1 over the first channel C1.

As shown in FIG. 8D, a plurality of source silicide regions SSR areformed in the first source S1 and the second source S2. As shown in FIG.8E, a source dielectric layer 140 is formed covering the first sourceS1, the second source S2, the source silicide regions SSR and theisolation portion 120. Subsequently, a gate dielectric layer 130 isformed surrounding the first channel C1, the second channel C2 and thethird channel C3. As shown in FIG. 8F, a high-k dielectric layer 150 anda gate G are formed over the source dielectric layer 140 and surroundingthe gate dielectric layer 130.

As shown in FIG. 8G, an ILD 160 is formed covering the firstchannel-drain structure CDS1, the second channel-drain structure CDS2,the third channel-drain structure CDS3 and the gate G, and aplanarization process is then performed to expose an upper surface ofthe first drain D1, an upper surface of the second drain D2 and an uppersurface of the third drain D3. Subsequently, a first drain pad DP1 and asecond drain pad DP2 are formed. The first drain pad DP1 is formed overand in contact with the first drain D1, and the second drain pad DP2 isformed over and in contact with the second drain D2 and the third drainD3.

After the formation of the first drain pad DP1 and the second drain padDP2, another ILD 210 is formed over the first drain pad DP1, the seconddrain pad DP2 and the ILD 160, as shown in FIG. 4. The ILDs 210, 160 arethen patterned to form openings, and a conductive material is thenfilled in the openings to form the conductive plugs P respectivelyconnected to the first drain pad DP1 and the second source S2.

The method of the present disclosure can be used to manufacture theseries-connected transistor structure for sharing Vds. Moreover, theseries-connected transistor structure manufactured by the method of thepresent disclosure can occupy a small area compared to aseries-connected transistor structure manufactured by a method includingforming metal lines and conductive plugs due to process limit of themetal lines and conductive plugs.

According to some embodiments, a series-connected transistor structureincludes a first source, a first channel-drain structure, a secondchannel-drain structure, a gate dielectric layer, a gate, a first drainpad and a second drain pad. The first source is over a substrate. Thefirst channel-drain structure is over the first source and includes afirst channel and a first drain over the first channel. The secondchannel-drain structure is over the first source and substantiallyparallel to the first channel-drain structure. The second channel-drainstructure includes a second channel and a second drain over the secondchannel. The gate dielectric layer surrounds the first channel and thesecond channel. The gate surrounds the gate dielectric layer. The firstdrain pad is over and in contact with the first drain. The second drainpad is over and in contact with the second drain, in which the firstdrain pad and the second drain pad are separated from each other.

According to some embodiments, a series-connected transistor structureincludes a first source, a second source, an isolation portion, a secondchannel-drain structure, a third channel-drain structure, a gatedielectric layer, a gate and a second drain pad. The first source isover a substrate. The second source is over the substrate and laterallyadjacent to the first source. The isolation portion is between the firstsource and the second source to electrically isolate the first sourcefrom the second source. The second channel-drain structure is over thefirst source and includes a second channel and a second drain over thesecond channel. The third channel-drain structure is over the secondsource and substantially parallel to the second channel-drain structure.The third channel-drain structure includes a third channel and a thirddrain over the third channel. The gate dielectric layer surrounds thesecond channel and the third channel. The gate surrounds the gatedielectric layer. The second drain pad is over and in contact with thesecond drain and the third drain.

According to some embodiments, a method of manufacturing aseries-connected transistor structure is provided, which includesforming a series-connected source-channel-drain structure protrudingover a substrate, the series-connected source-channel-drain structureincluding a first source over the substrate, a first channel-drainstructure over the first source and a second channel-drain structureover the first source, and the first channel-drain structure and thesecond channel-drain structure substantially parallel to each other, orincluding the first source, a second source over the substrate andlaterally adjacent to the first source, an isolation portion between thefirst source and the second source, the second channel-drain structureover the first source and a third channel-drain structure over thesecond source, and the second channel-drain structure and the thirdchannel-drain structure substantially parallel to each other. A sourcedielectric layer is formed over the first source, or over the firstsource and the second source. A gate dielectric layer is formedsurrounding a channel of the first channel-drain structure and a channelof the second channel-drain structure, or surrounding the channel of thesecond channel-drain structure and a channel of the third channel-drainstructure. A gate is formed over the source dielectric layer andsurrounding the gate dielectric layer. A first drain pad and a seconddrain pad are formed respectively over and in contact with a drain ofthe first channel-drain structure and a drain of the secondchannel-drain structure, or the second drain pad is formed over and incontact with the drain of the second channel-drain structure and a drainof the third channel-drain structure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A series-connected transistor structure,comprising: a first source over a substrate; a second source over thesubstrate and laterally adjacent to the first source; an isolationportion between the first source and the second source to electricallyisolate the first source from the second source; a second channel-drainstructure over the first source, the second channel-drain structurecomprising a second channel and a second drain over the second channel;a third channel-drain structure over the second source and substantiallyparallel to the second channel-drain structure, the third channel-drainstructure comprising a third channel and a third drain over the thirdchannel; a gate dielectric layer surrounding the second channel and thethird channel; a continuous gate surrounding the gate dielectric layer;and a second drain pad over and in contact with the second drain and thethird drain.
 2. The series-connected transistor structure of claim 1,wherein the continuous gate is between the second channel and the thirdchannel.
 3. The series-connected transistor structure of claim 1,further comprising two conductive plugs respectively connected to thefirst source and the second source.
 4. The series-connected transistorstructure of claim 1, further comprising a source dielectric layerbetween the first source and the continuous gate and between the secondsource and the continuous gate.
 5. The series-connected transistorstructure of claim 4, further comprising two conductive plugs, and thesource dielectric layer comprises two openings respectively exposing aportion of the first source and a portion of the second source, and thetwo conductive plugs are respectively in contact with the portion of thefirst source and the portion of the second source through the twoopenings.
 6. The series-connected transistor structure of claim 4,further comprising a high-k dielectric layer between the gate dielectriclayer and the continuous gate and between the source dielectric layerand the continuous gate.
 7. The series-connected transistor structure ofclaim 1, wherein the second drain pad comprises silicide.
 8. Theseries-connected transistor structure of claim 1, further comprising twosource silicide regions respectively in the first source and the secondsource.
 9. The series-connected transistor structure of claim 8, furthercomprising two conductive plugs respectively connected to the two sourcesilicide regions.
 10. The series-connected transistor structure of claim1, further comprising an inter-layer dielectric (ILD) covering thecontinuous gate and exposing an upper surface of the second drain and anupper surface of the third drain.
 11. The series-connected transistorstructure of claim 10, wherein the second drain pad is over the ILD. 12.The series-connected transistor structure of claim 10, wherein the ILDis between the second drain and the third drain.
 13. Theseries-connected transistor structure of claim 10, further comprisinganother ILD over the second drain pad.
 14. The series-connectedtransistor structure of claim 13, further comprising two conductiveplugs respectively connected to the first source and the second sourcethrough the ILD and the other ILD.
 15. A series-connected transistorstructure, comprising: a first source over a substrate; a second sourceover the substrate and separated from the first source; a secondchannel-drain structure over the first source, the second channel-drainstructure comprising a second channel and a second drain over the secondchannel; a third channel-drain structure over the second source andsubstantially parallel to the second channel-drain structure, the thirdchannel-drain structure comprising a third channel and a third drainover the third channel; a gate dielectric layer surrounding the secondchannel and the third channel; a continuous gate surrounding the gatedielectric layer; and a second drain pad over and in contact with thesecond drain and the third drain.
 16. The series-connected transistorstructure of claim 15, wherein the continuous gate is between the secondchannel and the third channel.
 17. The series-connected transistorstructure of claim 15, further comprising two conductive plugsrespectively connected to the first source and the second source.
 18. Aseries-connected transistor structure, comprising: a first source over asubstrate; a second source over the substrate and separated from thefirst source; a second channel-drain structure over the first source,and the second channel-drain structure comprising a second channel and asecond drain over the second channel; a third channel-drain structureover the second source and substantially parallel to the secondchannel-drain structure, and the third channel-drain structurecomprising a third channel and a third drain over the third channel,wherein the second drain is electrically connected to the third drain; agate dielectric layer surrounding the second channel and the thirdchannel; and a continuous gate surrounding the gate dielectric layer.19. The series-connected transistor structure of claim 18, wherein thecontinuous gate is between the second channel and the third channel. 20.The series-connected transistor structure of claim 18, furthercomprising two conductive plugs respectively connected to the firstsource and the second source.